Fixed load target with installed rdoc. I enforce to use minitest_helper on repository.

Author: hsbt

lib/rdoc.rb

@@ -125,8 +125,6 @@ def self.load_yaml
 
   autoload :RDoc,           'rdoc/rdoc'
 
-  autoload :TestCase,       'rdoc/test_case'
-
   autoload :CrossReference, 'rdoc/cross_reference'
   autoload :ERBIO,          'rdoc/erbio'
   autoload :ERBPartial,     'rdoc/erb_partial'

test/xref_test_case.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 ENV['RDOC_TEST'] = 'yes'
 
-require 'rdoc'
+require 'minitest_helper'
 require File.expand_path '../xref_data', __FILE__
 
 class XrefTestCase < RDoc::TestCase